G -Code Movement

Hi
I know that if I want to move a axis to a postion I use : G1 z8 and it will go to the position 8mm ( Mendel90 ) from the bed.
However how can I move an axis 100mm in distance and not absolute postion.

I want to measure the correct distance my axis are moving.

I know I can use Pronterface dials.

Thanks

See [reprap.org].

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].